The role of glaciers as storages of water resources is of primary importance in arid and semiarid mountain regions characterised by pronounced seasonality and long melting periods without significant precipitation. A robust understanding of the physical processes controlling the energy and mass balance of glaciers in these areas is thus crucial for estimation of water resources and their seasonal variability, long term trends and future changes.
It has been suggested that glaciers in dry regions are strongly affected by mass and energy losses associated to surface sublimation, but few studies provide a quantification of its influence at the glacier or catchment scales.
The main objective of this thesis is to understand and quantify the role of surface sublimation in the energy and mass balance of glaciers in dry environments, by means of point-scale and distributed physically-based energy and mass balance models. Complementary to this objective, this thesis also addresses three relevant topics in semiarid catchments related to the hydrological role of debris-free glaciers and the modelling of glacier ablation.
These topics are the runoff contribution of debris-free glaciers in comparison to that of the seasonal snowpack and debris-covered glaciers, the use of temperature-index models under sublimation-favourable conditions and the spatial distribution of near-surface air temperature over mountain glaciers during the ablation season.
As study region, the subtropical semiarid Andes of North-Central Chile (29-34°S) is selected. This region is characterized by elevations up to almost 7000 m a.s.l., a dry environment, a large number of debris-free and debris-covered glaciers and a strong dependence of ecosystems and human activities on fresh water resources originated from snow and ice melt. In this region, it has been suggested that distributed, physically-based models at the glacier and catchment scales can bridge the gap between regional studies based on remote sensing products and specific studies focused on point-scale process understanding. The presented analyses are conducted on seven glaciers for which a unique dataset of meteorological, glaciological and hydrological variables has been collected in the period 2008-2015. The study sites are grouped in two clusters, one in North Chile (29-30°S), in which climatic conditions are particularly dry, and another one in Central Chile (32-34°S), where the climate is more Mediterranean.
To achieve the proposed objectives, the collected field data is analysed and complemented with remote sensing products to force several hydrological, melt and energy balance models at different spatial scales. The main results of these analyses can be summarised as follows: i) At the integrated glacier scale, surface sublimation accounts for 6.6\% of total ablation during a 2-month summer period in a selected case study in the Juncal Norte Glacier (33°S). It is found that surface sublimation is negligible in comparison to melt at low-elevation low-albedo sites, but it dominates at high-elevation wind-exposed sites, where it represents most of the total ablation. Negative latent heat fluxes, associated with sublimation, are one of the largest sinks in the glacier energy balance and consistently reduce the energy available for melt. ii) Despite having remarkably different spatial and temporal mass balances patterns, the total annual contribution to runoff of low-elevation debris-covered glaciers is similar to that of high-elevation debris-free glaciers. iii) At low-elevation sites, the performance of an enhanced temperature-index model is good compared to that of an energy balance model and its parameters are transferable from one glacier to another and from season to season. However, its performance and parameter transferability tends to decrease with elevation as energy losses modify the diurnal cycle of surface temperature and lower the correlation of melt and index variables. iv) During warm periods, the most relevant controls of near-surface air temperature over glaciers are off-glacier lapse rates and the advection of cold air by katabatic winds and warm air by up-valley winds. A new air temperature distribution model including these processes has been developed and tested with positive results.
In relation to the main research question of this thesis, it is concluded that neglecting surface sublimation in the glacier mass balance has relatively small consequences in the simulation of one ablation season, but it might have large cumulative effects in long term simulations, especially on glaciers in very dry environments, such as Tapado and Guanaco glaciers in North-Chile. Physically-based (or oriented) distributed models, as the ones presented in this study, have the potential to shed light on the dominant processes and energy and mass fluxes on glacierised catchments, and their spatial and temporal patterns. However, much work needs to be committed to i) develop model components that reproduce still poorly-known processes, such as turbulent fluxes on penitente fields or ablation processes on debris-covered glaciers, ii) generate appropriate meteorological forcing fields, and iii) obtain the corresponding on-site model validation data. The obtained results and developed methods are likely to be relevant for the scientific community of glaciologists and hydrologists, and to communities, decision-makers, water-managers and engineers interested on glaciers and water resources in arid and semiarid regions, particularly those in the central regions of Chile and ArgentinaShow more